1. Field of the Invention
This invention relates to a radiation image read-out apparatus comprising a read-out section and an erasing section combined integrally for use in a radiation image recording and reproducing system. This invention particularly relates to a radiation image read-out apparatus wherein the processing time for a plurality of stimulable phosphor sheets can be shortened.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Patent Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store a radiation image until the sheet is scanned with stimulating rays to read out the radiation image. Therefore, after the radiation image is read out from the stimulable phosphor sheet, radiation energy remaining thereon should be erased before reusing the sheet.
For satisfying this requirement, it has been proposed to provide a radiation image read-out apparatus with a read-out section for reading out an image stored on a stimulable phosphor sheet, and an erasing section for erasing radiation energy remaining on the sheet.
In the proposed radiation image read-out apparatus, stimulable phosphor sheets carrying radiation images stored thereon are fed as housed in a sheet housing means such as a cassette for housing a single sheet or a magazine for housing a plurality of sheets. The stimulable phosphor sheets are then taken one by one out of the sheet housing means, and sequentially conveyed to the read-out section and the erasing section. At the read-out section, preliminary read-out is often carried out for approximately ascertaining the radiation image stored on the stimulable phosphor sheet before the image read-out (final read-out) is conducted for obtaining the image signal for use in reproduction of a visible image. Read-out conditions in the final read-out and/or other conditions are adjusted based on the information obtained by the preliminary read-out.
On the other hand, stimulable phosphor sheets which need not be subjected to the image read-out (final read-out), for example sheets on which image recording was carried out incorrectly, or the sheets other than those carrying a desired image stored thereon among the sheets on which image recording was carried out in different conditions, are often included among the sheets fed to the radiation image read-out apparatus. Particularly in the case where digital subtraction processing of radiation images is carried out, many stimulable phosphor sheets which need not be subjected to the final read-out are often fed to the radiation image read-out apparatus.
In the method of digital subtraction processing of radiation images, two radiation images recorded under conditions different from each other are photoelectrically read out to obtain digital image signals, which are then subjected to subtraction processing with respect to the corresponding picture elements of the images, thereby to obtain a difference signal for forming an image of a specific structure contained in the radiation images. This method makes it possible to reproduce a radiation image of only the specific structure by use of the signal thus obtained Basically, subtraction processing is carried out by either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, the image of a specific structure is extracted by subtracting the image signal of a radiation image obtained without injection of contrast media from the image signal of a radiation image in which the image of the specific structure is enhanced by the injection of a contrast medium. In the latter method, an object is exposed to radiations having energy distributions different from each other to obtain two radiation images each containing the image of a specific structure recorded on the basis of the intrinsic radiation energy absorption characteristics of the specific structure. Then, the image signals of the two radiation images are weighted appropriately, when necessary, and subjected to subtraction to extract the image of the specific structure. Since subtraction processing is extremely effective for diagnostic purposes in image processings for medical X-ray photographs, it has attracted much attention in recent years, and research has continued to develop improved methods by use of electronic technology.
Subtraction processing wherein stimulable phosphor sheets are used may be carried out by the method as proposed in, for example, Japanese Unexamined patent Publication No. 58(1983)-163340. The proposed method comprises the steps of (i) using two or more stimulable phosphor sheets exhibiting an extremely wide latitude of exposure to a radiation, (ii) exposing the stimulable phosphor sheets to the radiation passing through the same object under different conditions, i.e. with and without the injection of contrast media to the object, to have radiation images of the object stored on the stimulable phosphor sheets, image information of the object portion injected with contrast media being different between the radiation images, (iii) detecting the radiation images by scanning the stimulable phosphor sheets with stimulating rays to obtain digital image signals, and (iv) carrying out digital subtraction processing by use of the digital image signals.
When the aforesaid subtraction processing is carried out, the image recording portion of the object is injected with contrast media in the course of image recording, and image recording is conducted many times as the contrast media diffuse through the image recording portion of the object. A plurality of the stimulable phosphor sheets each carrying a radiation image stored thereon in this manner are housed in a magazine or the like, fed to the radiation image read-out apparatus, and sent to the image read-out step one by one. In this case, only the stimulable phosphor sheet carrying a radiation image stored thereon before injection of contrast media and the stimulable phosphor sheet carrying a radiation image stored thereon after contrast media have diffused in the manner most suitable for carrying out subtraction processing are actually used for subtraction processing, and the other sheets are unnecessary for subtraction processing. Therefore, it is necessary to select the two stimulable phosphor sheets which are to be used for subtraction processing from many sheets on which image recording was conducted. For this purpose, all of the radiation images stored on the stimulable phosphor sheets must be reproduced into visible images. However, with the conventional radiation image read-out apparatus wherein the image-recorded stimulable phosphor sheets are sent one by one to the read-out section and the preliminary read-out and the final read-out are conducted on the respective sheets, the two sheets which are to be used for subtraction processing cannot be selected until the final read-out is finished for all of the sheets, and a long time is taken for selecting the two desired sheets. Also, since the final read-out is carried out also for the sheets which are unnecessary for subtraction processing, much energy is consumed unnecessarily for producing stimulating rays or the like.